I have used Mameplus! for several years.
There "were" several advantages using Mameplus! over standard MAME, but many of the advantages have diminished over years....
Advantages (still effective)
#1. Image enhancement filters like HQx, xBRZ filters
#2. Command.dat support (In-game display of fighting game moves)
#3. Removing nag screen
Advantages (now obsolete)
#1. It came with separate GUI called M+GUI, which was much faster than MAME with integrated GUI.
-> But not faster than attract-mode.

#2. It combined MAME/MESS with single executable (long before devs permitted this)
-> Now it is officially combined by standard MAME...
#3. Sometimes, controversal ROMs are dropped by mainstream MAME, due to various reasons. (For example, Cave SH3 driver). But derivative builds, including Mameplus! tends to ignore such decisions...
-> But the Cave SH3 driver is back to mainstream MAME again, so it doesn't matter again..
...lastly, I'd like to mention that many derivative MAME builds do not describe their difference to the mainstream MAMEs. (What is Ash-build, anyway?). It would be really helpful to have feature-comparison summary on derivative builds.